Digital video disc (DVD) devices store audio-visual data in a highly compressed form and play the audio-visual data to a user. These devices have a read only memory (ROM). The DVD CD-ROM disc is a super-density disc that can hold up to 18 gigabytes of audio, video and other types of data (e.g., menus, sub-pictures, graphics, etc.). As part of the audio-visual data, the DVD devices store video images on the disc so that the images may be later recalled and displayed on a video display. DVD CD-ROM players retrieve and display video images that have been compressed under known video compression techniques like the International Standard Organization's (ISO) Moving Picture Expert Group (MPEG) techniques MPEG 1 and MPEG 2.
MPEG 1 is an ISO standard defined in ISO/IEC 11172 that sets forth a standard format for storing and distributing audio and motion video. Some of the features of MPEG 1 include random access, fast forward, and reverse playback. Consequently, MPEG 1 has been used as the basis for video CDs and many video games. The goal of MPEG 1 is playback of digital audio and video using a standard compact disk with a bit rate of 1.416 Mbps, where 1.15 Mbps of this bit rate is designated for video.
MPEG 2 extends MPEG 1 to cover a wider range of applications. MPEG 2 is an ISO standard as defined by ISO/IEC 13818. The primary application originally targeted by MPEG 2 was all-digital transmission of broadcast-quality video at bit rates of 4-9 Mbps. However, MPEG 2 has become useful for many other applications, such as high definition television, and MPEG 2 now supports bit rates of 1.5-60 Mbps.
In addition to playing video images, DVD devices can also read and play compressed audio sequences using known audio decompression techniques (e.g., Dolby AC3, MPEG 1 or MPEG 2). As such, these systems are especially well-suited for playing audio-visual works, such as movies.
When playing an audio-visual work like a movie, the DVD device reads an audio-visual stream from the DVD CD-ROM and displays the video portion of the stream on the video display and plays the audio portion of the stream on a speaker. To facilitate the playing of the stream, the stream is stored on the CD-ROM using time stamps from a 27 MHz clock to indicate when a particular portion of the stream is to be played. These time stamps from the 27 MHz clock are also used to synchronize the audio and video portion of the stream at playtime. Otherwise, if the audio and video portion fell out of synchronization, the quality of the performance of the audio-visual work would greatly suffer (the viewer would notice a loss of lip synchronization). As a result of the time stamps of the audio-visual stream being generated using a 27 MHz clock, a clock running at 27 MHz is needed at playtime by the DVD device to ensure both that portions of the steam are played at the appropriate time and that the audio portion and the video portion of the stream are synchronized. As such, DVD playback devices have a 27 MHz clock. It should be noted that audio-visual streams encoded using the AC3 standard, the MPEG 1 standard, and the MPEG 2 standard use a 27 MHz clock for synchronization.
Although DVD devices have been developed, they are typically stand-alone devices and have not been integrated with other systems. However, by integrating the functionality of a DVD device with a personal computer, additional functionality can be provided to a user. It is therefore desirable to integrate a DVD device into an existing system like a personal computer.